Liquid crystal phases which form a helix structure with a given direction of rotation are increasingly required for liquid crystal displays. Thus, materials of this type are required, for example, for the Schadt-Helfrich effect in order to avoid the undesirable effect of "reverse twist" (E. Guyon and W. Urbach in "Nonemissive Electrooptic Displays", edited by A. R. Kmetz, F. K. von Willisen, Plenum Press, New York-London, 1976, page 127), for the cholesteric-nematic phase transition effect and for bistability effects.
An important problem here is the production of a suitable temperature function of the helix pitch, which depends on the particular electrooptical effect and its particular embodiment.
For example, for liquid crystal display elements based on the twisted nematic cell, a temperature-independent pitch is suitable for avoiding "reverse twist". Furthermore, it has been possible to show that compensation of the temperature drift of the threshold voltage of a twisted nematic cell can be achieved if the helix pitch decreases as the temperature increases (P. R. Gerber, Physics Letters 78A, 285 (1980)). Similarly, for the phase transition effect, compensation of the threshold voltage drift is achieved by a helix pitch which decreases greatly as the temperature increases (A. Gobl-Wunsch, G. Heppke and F. Oestreicher, Journal de Physique 40, 773 (1979)).
The liquid crystal phases used for this purpose generally consist of mixtures of non-chiral liquid crystal compounds to which chiral compounds are added to produce the helix structure. Virtually all the known chiral doping substances induce helix structures, the pitches of which increase to a greater or lesser degree over wide ranges as the temperature increases. In the literature, only certain spirobiindane derivatives with a negative temperature function gradient are reported (Advances of Infrared and Raman Spectroscopy 8 (1981) Chapter 4). However, in practice, it has not been possible to eliminate the troublesome temperature drift with these compounds. It has hitherto been possible to achieve the negative temperature function gradient frequently desired only by using two suitable doping substances of different direction of rotation and different relative temperature dependency (German . . . No. 2,827,471). The disadvantages of this multiple-doping process are, inter alia, the maintenance of the exact concentration ratio of the two chiral compounds and the restriction to a limited temperature range as well as the high overall concentration required for the doping substances (A. Gobl-Wunsch, G. Heppke and F. Oestreicher, Journal de Physique, 40, 773 (1979)). Outdoor applications are therefore impossible.